LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator FEATURES DESCRIPTION ■ Wide Input Voltage Range: 3.6V to 25V The LT®1938 is an adjustable frequency (300kHz to ■ 2.2A Maximum Output Current 2.8MHz) monolithic buck switching regulator that accepts ■ Adjustable Switching Frequency: 300kHz to 2.8MHz input voltages up to 25V. A high efficiency 0.18Ω switch ■ Low Shutdown Current: I < 1µA is included on the die along with a boost Schottky diode Q ■ Integrated Boost Diode and the necessary oscillator, control and logic circuitry. ■ Power Good Flag Current mode topology is used for fast transient response ■ Saturating Switch Design: 0.18Ω On-Resistance and good loop stability. The LT1938’s high operating fre- ■ 1.265V Feedback Reference Voltage quency allows the use of small, low cost inductors and ■ Output Voltage: 1.265V to 20V ceramic capacitors resulting in low output ripple while ■ Soft-Start Capability keeping total solution size to a minimum. The low current ■ Small 10-Pin Thermally Enhanced (3mm × 3mm) shutdown mode reduces input supply current to less than DFN Package 1µA while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). A APPLICATIONS power good flag signals when V reaches 90% of the OUT programmed output voltage. The LT1938 is available in ■ Automotive Battery Regulation a 3mm × 3mm DFN package with Exposed Pad for low ■ Power for Portable Products thermal resistance. ■ Distributed Supply Regulation ■ Industrial Supplies , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. ■ Wall Transformer Regulation TYPICAL APPLICATION 3.3V Step-Down Converter Efficiency (V = 3.3V) OUT VIN VOUT 4.5V TO 3.3V 90 25V 2.2A VIN = 7V VIN BD 85 OFF ON RUN/SS BOOST 80 16.2k 0.47µF 4.7µH VIN = 24V VIN = 12V 4.7µF VC LT1938 SW Y (%) 75 C RT N 70 680pF CIE PG BIAS FFI 65 324k E 60.4k GND FB 60 200k 22µF 55 L: NEC PLC-0745-4R7 f = 800kHz 50 1938 TA01 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 LOAD CURRENT (A) 1938 G02 1938fa 1

LT1938 AbSOLUTE MAxIMUM RATINgS PIN CONFIgURATION (Note 1) V , RUN/SS Voltage .................................................25V IN TOP VIEW BOOST Pin Voltage ...................................................50V BOOST Pin Above SW Pin .........................................25V BD 1 10 RT BOOST 2 9 VC FB, RT, V Voltage .......................................................5V C SW 3 11 8 FB BIAS, PG, BD Voltage ................................................25V VIN 4 7 BIAS Operating Junction Temperature Range (Note 2) RUN/SS 5 6 PG LT1938E .............................................–40°C to 125°C DD PACKAGE LT1938I ..............................................–40°C to 125°C 10-LEAD (3mm × 3mm) PLASTIC DFN Storage Temperature Range ...................–65°C to 150°C TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT1938EDD#PBF LT1938EDD#TRPBF LDFT 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C LT1938IDD#PBF LT1938IDD#TRPBF LDFT 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ELECTRICAL ChARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T = 25°C. V = 10V, V = 10V, V = 15V, V = 3.3V unless otherwise A IN RUN/SS BOOST BIAS noted. (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Input Voltage ● 3 3.6 V Quiescent Current from V V = 0.2V 0.01 0.5 µA IN RUN/SS V = 3V, Not Switching ● 0.4 0.8 mA BIAS V = 0, Not Switching 1.2 2.0 mA BIAS Quiescent Current from BIAS V = 0.2V 0.01 0.5 µA RUN/SS V = 3V, Not Switching ● 0.85 1.5 mA BIAS V = 0, Not Switching 0 0.1 mA BIAS Minimum Bias Voltage 2.7 3 V Feedback Voltage 1.25 1.265 1.28 V ● 1.24 1.265 1.29 V FB Pin Bias Current (Note 3) ● 30 100 nA FB Voltage Line Regulation 4V < V < 25V 0.002 0.02 %/V IN 1938fa 2

LT1938 ELECTRICAL ChARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T = 25°C. V = 10V, V = 10V V = 15V, V = 3.3V unless otherwise A IN RUN/SS BOOST BIAS noted. (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS Error Amp g 330 µMho m Error Amp Gain 1000 V Source Current 75 µA C V Sink Current 100 µA C V Pin to Switch Current Gain 3.5 A/V C V Clamp Voltage 2 V C Switching Frequency R = 8.66k 2.7 3.0 3.3 MHz T R = 29.4k 1.25 1.4 1.55 MHz T R = 187k 250 300 350 kHz T Minimum Switch Off-Time ● 100 150 nS Switch Current Limit Duty Cycle = 5% 3.1 3.6 4.0 A Switch V I = 2A 360 mV CESAT SW Boost Schottky Reverse Leakage V = 10V, V = 0V 0.02 2 µA SW BIAS Minimum Boost Voltage (Note 4) ● 1.6 2.1 V BOOST Pin Current I = 1A 18 30 mA SW RUN/SS Pin Current V = 2.5V 5 10 µA RUN/SS RUN/SS Input Voltage High 2.5 V RUN/SS Input Voltage Low 0.2 V PG Threshold Offset from Feedback Voltage V Rising 100 mV FB PG Hysteresis 10 mV PG Leakage V = 5V 0.1 1 µA PG PG Sink Current V = 0.4V ● 100 300 µA PG Note 1: Stresses beyond those listed under Absolute Maximum Ratings Note 3: Bias current measured in regulation. Bias current flows into the FB may cause permanent damage to the device. Exposure to any Absolute pin. Maximum Rating condition for extended periods may affect device Note 4: This is the minimum voltage across the boost capacitor needed to reliability and lifetime. guarantee full saturation of the switch. Note 2: The LT1938E is guaranteed to meet performance specifications from 0°C to 125°C. Specifications over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT1938I specifications are guaranteed over the –40°C to 125°C temperature range. 1938fa 3

LT1938 TYPICAL PERFORMANCE ChARACTERISTICS (T = 25°C unless otherwise noted) A Efficiency (V = 5.0V) Efficiency (V = 3.3V) Efficiency OUT OUT 100 90 90 VIN = 7V VIN = 12V VIN = 12V 85 85 90 80 80 Y (%) 80 VIN = 24V Y (%) 75 VIN = 24V VIN = 12V Y (%) 75 VIN = 24V C C C N N 70 N 70 CIE CIE CIE EFFI 70 EFFI 65 EFFI 65 60 60 60 L: NEC PLC-0745-4R7 55 L: NEC PLC-0745-4R7 55 VL O=U 1T0 =µ H3.3V f = 800kHz f = 800kHz LOAD = 1A 50 50 50 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) LOAD CURRENT (A) SWITCHING FREQUENCY (MHz) 1938 G01 1938 G02 1938 G03 Maximum Load Current Maximum Load Current Switch Current Limit 4.0 4.0 4.0 3.5 3.5 3.5 TYPICAL A) TYPICAL T ( D CURRENT (A) 23..50 MINIMUM D CURRENT (A) 23..50 MINIMUM CURRENT LIMI 23..50 LOA 2.0 LOA 2.0 TCH 2.0 1.5 VTAO U=T 2 =5 °3C.3V 1.5 VTAO U=T 2 =5 °5CV SWI 1.5 L = 4.7µH L = 4.7µH f = 800kHz f = 800kHz 1.0 1.0 1.0 5 10 15 20 25 5 10 15 20 25 0 20 40 60 80 100 INPUT VOLTAGE (V) INPUT VOLTAGE (V) DUTY CYCLE (%) 1938 G04 1938 G05 1938 G06 Switch Current Limit Switch Voltage Drop Boost Pin Current 4.5 700 90 4.0 DUTY CYCLE = 10 % 600 80 NT LIMIT (A) 23..50 DUTY CYCLE = 90 % OP (mV) 450000 RENT (mA) 567000 E R R R 2.0 D U SWITCH CUR 11..50 VOLTAGE 320000 BOOST PIN C 243000 0.5 100 10 0 0 0 –50 –25 0 25 50 75 100 125 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 TEMPERATURE (°C) SWITCH CURRENT (mA) SWITCH CURRENT (mA) 1938 G07 1938 G08 1938 G09 1938fa 4

LT1938 TYPICAL PERFORMANCE ChARACTERISTICS (T = 25°C unless otherwise noted) A Feedback Voltage Switching Frequency Frequency Foldback 1.290 1.20 1200 RT = 45.3k RT = 45.3k 1.285 1.15 Hz) 1000 ACK VOLTAGE (V)111...222787005 QUENCY (MHz) 111...010005 G FREQUENCY (k 680000 B1.265 E 0.95 N FEED1.260 FR 0.90 WITCHI 400 S 200 1.255 0.85 1.250 0.80 0 –50 –25 0 25 50 75 100 125 –50 –25 0 25 50 75 100 125 0 200 400 600 800 1000 1200 1400 TEMPERATURE (°C) TEMPERATURE (°C) FB PIN VOLTAGE (mV) 1938 G10 1938 G11 1938 G12 Minimum Switch On-Time Soft-Start RUN/SS Pin Current 140 4.0 12 s) 120 3.5 10 MINIMUM SWITCH ON-TIME (n 18642000000 SWITCH CURRENT LIMIT (A) 120321......005055 RUN/SS PIN CURRENT (µA) 4628 0 0 0 –50 –25 0 25 50 75 100 125 0 0.5 1 1.5 2 2.5 3 3.5 0 5 10 15 20 25 TEMPERATURE (°C) RUN/SS PIN VOLTAGE (V) RUN/SS PIN VOLTAGE (V) 1938 G13 1938 G14 1938 G15 Boost Diode Error Amp Output Current Minimum Input Voltage 1.6 100 4.5 1.4 80 4.0 60 1.2 E V (V)f 1.0 NT (µA) 40 GE (V) 3.5 BOOST DIOD 000...864 VPIN CURREC –22000 INPUT VOLTA 3.0 –40 2.5 VOUT = 3.3V TA = 25°C 0.2 –60 L = 4.7µH f = 800kHz 0 –80 2.0 0 0.5 1.0 1.5 2.0 1.065 1.165 1.265 1.365 1.465 0.001 0.01 0.1 1 10 BOOST DIODE CURRENT (A) FB PIN VOLTAGE (V) LOAD CURRENT (A) 1938 G16 1938 G17 1938 G18 1938fa 5

LT1938 TYPICAL PERFORMANCE ChARACTERISTICS (T = 25°C unless otherwise noted) A Minimum Input Voltage V Voltages Power Good Threshold C 6.5 2.50 1.200 PG RISING 6.0 2.00 1.180 V) V) GE (V) 5.5 LTAGE ( 1.50 CURRENT LIMIT CLAMP LTAGE ( 1.160 A O O OLT D V D V V L L INPUT 5.0 HRESHO 1.00 SWITCHING THRESHOLD HRESHO1.140 4.5 VOUT = 5V T 0.50 T 1.120 TA = 25°C L = 4.7µH f = 800kHz 4.0 0 1.100 0.001 0.01 0.1 1 10 –50 –25 0 25 50 75 100 125 –50 –25 0 25 50 75 100 125 LOAD CURRENT (A) TEMPERATURE (°C) TEMPERATURE (°C) 1938 G19 1938 G20 1938 G21 Switching Waveforms Switching Waveforms (Discontinuous Operation) (Continuous Operation) IL IL 0.5A/DIV 0.5A/DIV VSW VSW 5V/DIV 5V/DIV VOUT VOUT 10mV/DIV 10mV/DIV VIN = 12V, FRONT PAGE APPLICATION VIN = 12V, FRONT PAGE APPLICATION ILOAD = 140mA ILOAD = 1A 1µs/DIV 1938 G22 1µs/DIV 1938 G23 1938fa 6

LT1938 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost PG (Pin 6): The PG pin is the open collector output of an Schottky diode. internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG output is BOOST (Pin 2): This pin is used to provide a drive valid when V is above 3.5V and RUN/SS is high. voltage, higher than the input voltage, to the internal bipolar IN NPN power switch. BIAS (Pin 7): The BIAS pin supplies the current to the LT1938’s internal regulator. Tie this pin to the lowest SW (Pin 3): The SW pin is the output of the internal power available voltage source above 3V (typically V ). This switch. Connect this pin to the inductor, catch diode and OUT architecture increases efficiency especially when the input boost capacitor. voltage is much higher than the output. V (Pin 4): The V pin supplies current to the LT1938’s IN IN FB (Pin 8): The LT1938 regulates the FB pin to 1.265V. internal regulator and to the internal power switch. This Connect the feedback resistor divider tap to this pin. pin must be locally bypassed. V (Pin 9): The V pin is the output of the internal error RUN/SS (Pin 5): The RUN/SS pin is used to put the C C amplifier. The voltage on this pin controls the peak switch LT1938 in shutdown mode. Tie to ground to shut down current. Tie an RC network from this pin to ground to the LT1938. Tie to 2.3V or more for normal operation. If compensate the control loop. the shutdown feature is not used, tie this pin to the V IN pin. RUN/SS also provides a soft-start function; see the RT (Pin 10): Oscillator Resistor Input. Connecting a resistor Applications Information section. to ground from this pin sets the switching frequency. Exposed Pad (Pin 11): Ground. The Exposed Pad must be soldered to the PCB. 1938fa 7

LT1938 bLOCK DIAgRAM VIN 4 VIN C1 BIAS 7 INTERNAL 1.265V REF – BD + 1 RUN/SS 5 ∑ SLOPE COMP SWITCH BOOST LATCH 2 C3 R 10 RT OSCILLATOR Q 300kHz–2.8MHz S RT SW 3 L1 VOUT SOFT-START VC CLAMP D1 C2 PG 6 ERROR AMP + 1.12V + VC 9 – – CC CF GND FB RC 11 8 R2 R1 1938 BD 1938fa 8

LT1938 OPERATION The LT1938 is a constant frequency, current mode step- This improves efficiency. The RUN/SS pin is used to place down regulator. An oscillator, with frequency set by RT, the LT1938 in shutdown, disconnecting the output and enables an RS flip-flop, turning on the internal power reducing the input current to less than 1µA. switch. An amplifier and comparator monitor the current The switch driver operates from either the input or from flowing between the V and SW pins, turning the switch IN the BOOST pin. An external capacitor and diode are used off when this current reaches a level determined by the to generate a voltage at the BOOST pin that is higher than voltage at V . An error amplifier measures the output C the input supply. This allows the driver to fully saturate voltage through an external resistor divider tied to the FB the internal bipolar NPN power switch for efficient pin and servos the V pin. If the error amplifier’s output C operation. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the The oscillator reduces the LT1938’s operating frequency V pin provides current limit. The V pin is also clamped to when the voltage at the FB pin is low. This frequency C C the voltage on the RUN/SS pin; soft-start is implemented foldback helps to control the output current during startup by generating a voltage ramp at the RUN/SS pin using an and overload. external resistor and capacitor. The LT1938 contains a power good comparator which trips An internal regulator provides power to the control cir- when the FB pin is at 90% of its regulated value. The PG cuitry. The bias regulator normally draws power from the output is an open-collector transistor that is off when the V pin, but if the BIAS pin is connected to an external output is in regulation, allowing an external resistor to pull IN voltage higher than 3V bias power will be drawn from the the PG pin high. Power good is valid when the LT1938 is external source (typically the regulated output voltage). enabled and VIN is above 3.6V. 1938fa 9

LT1938 APPLICATIONS INFORMATION FB Resistor Network where V is the typical input voltage, V is the output IN OUT voltage, is the catch diode drop (~0.5V), V is the internal The output voltage is programmed with a resistor divider SW switch drop (~0.5V at max load). This equation shows between the output and the FB pin. Choose the 1% resis- that slower switching frequency is necessary to safely tors according to: accommodate high V /V ratio. Also, as shown in IN OUT ⎛ V ⎞ the next section, lower frequency allows a lower dropout R1=R2 OUT –1 ⎝⎜1.265 ⎠⎟ voltage. The reason input voltage range depends on the switching frequency is because the LT1938 switch has Reference designators refer to the Block Diagram. finite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of Setting the Switching Frequency ~150ns. This means that the minimum and maximum The LT1938 uses a constant frequency PWM architecture duty cycles are: that can be programmed to switch from 300kHz to 2.8MHz DC =f t MIN SW ON(MIN) by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching DCMAX =1–fSWtOFF(MIN) frequency is in Figure 1. where f is the switching frequency, the t is the SW ON(MIN) SWITCHING FREQUENCY (MHz) R VALUE (kΩ) minimum switch on time (~150ns), and the t is T OFF(MIN) 0.2 267 the minimum switch off time (~150ns). These equations 0.3 187 show that duty cycle range increases when switching 0.4 133 0.6 84.5 frequency is decreased. 0.8 60.4 1.0 45.3 A good choice of switching frequency should allow ad- 1.2 36.5 equate input voltage range (see next section) and keep 1.4 29.4 the inductor and capacitor values small. 1.6 23.7 1.8 20.5 2.0 16.9 Input Voltage Range 2.2 14.3 2.4 12.1 The maximum input voltage for LT1938 applications de- 2.6 10.2 pends on switching frequency, the Absolute Maximum Rat- 2.8 8.66 ings on V and BOOST pins, and on operating mode. IN Figure 1. Switching Frequency vs R Value T If the output is in start-up or short-circuit operating modes, Operating Frequency Tradeoffs then V must be below 25V and below the result of the IN Selection of the operating frequency is a tradeoff between following equation: efficiency, component size, minimum dropout voltage, and V +V OUT D maximum input voltage. The advantage of high frequency V = –V +V IN(MAX) D SW f t operation is that smaller inductor and capacitor values may SW ON(MIN) be used. The disadvantages are lower efficiency, lower where V is the maximum operating input voltage, IN(MAX) maximum input voltage, and higher dropout voltage. The V is the output voltage, V is the catch diode drop OUT D highest acceptable switching frequency (f ) for a SW(MAX) (~0.5V), V is the internal switch drop (~0.5V at max SW given application can be calculated as follows: load), f is the switching frequency (set by R ), and SW T V +V t is the minimum switch on time (~150ns). Note that D OUT ON(MIN) f = SW(MAX) t (V +V –V ) a higher switching frequency will depress the maximum ON(MIN) D IN SW operating input voltage. Conversely, a lower switching 1938fa 10

LT1938 APPLICATIONS INFORMATION frequency will be necessary to achieve safe operation at at least 3.5A at low duty cycles and decreases linearly to high input voltages. 2.5A at DC = 0.8. The maximum output current is a func- tion of the inductor ripple current: If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to I = I – ∆I /2 OUT(MAX) LIM L 25V are acceptable regardless of the switching frequency. Be sure to pick an inductor ripple current that provides In this mode, the LT1938 may enter pulse skipping opera- sufficient maximum output current (I ). OUT(MAX) tion where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple The largest inductor ripple current occurs at the highest and inductor current ripple will be higher than in normal VIN. To guarantee that the ripple current stays below the operation. specified maximum, the inductor value should be chosen according to the following equation: The minimum input voltage is determined by either the LT1938’s minimum operating voltage of ~3.6V or by its ⎛ ⎞ ⎛ V +V ⎞ V +V maximum duty cycle (see equation in previous section). L= OUT D ⎜1– OUT D⎟ ⎜ ⎟ The minimum input voltage due to duty cycle is: ⎝ fΔIL ⎠⎝⎜ VIN(MAX) ⎠⎟ V +V V = OUT D –V +V where VD is the voltage drop of the catch diode (~0.4V), IN(MIN) D SW 1–fSWtOFF(MIN) VIN(MAX) is the maximum input voltage, VOUT is the output voltage, f is the switching frequency (set by R ), and L SW T where V is the minimum input voltage, and t IN(MIN) OFF(MIN) is in the inductor value. is the minimum switch off time (150ns). Note that higher The inductor’s RMS current rating must be greater than the switching frequency will increase the minimum input maximum load current and its saturation current should be voltage. If a lower dropout voltage is desired, a lower about 30% higher. For robust operation in fault conditions switching frequency should be used. (start-up or short circuit) and high input voltage (>30V), Inductor Selection the saturation current should be above 3A. To keep the efficiency high, the series resistance (DCR) should be less For a given input and output voltage, the inductor value than 0.1Ω, and the core material should be intended for and switching frequency will determine the ripple current. high frequency applications. Table 1 lists several vendors The ripple current ∆I increases with higher V or V L IN OUT and suitable types. and decreases with higher inductance and faster switch- ing frequency. A reasonable starting point for selecting Table 1. Inductor Vendors the ripple current is: VENDOR URL PART SERIES TYPE Murata www.murata.com LQH55D Open ∆I = 0.4(I ) L OUT(MAX) TDK www.componenttdk.com SLF7045 Shielded where I is the maximum output load current. To SLF10145 Shielded OUT(MAX) guarantee sufficient output current, peak inductor current Toko www.toko.com D62CB Shielded must be lower than the LT1938’s switch current limit (I ). D63CB Shielded LIM The peak inductor current is: D75C Shielded D75F Open I = I + ∆I /2 L(PEAK) OUT(MAX) L Sumida www.sumida.com CR54 Open where I is the peak inductor current, I is CDRH74 Shielded L(PEAK) OUT(MAX) the maximum output load current, and ∆I is the inductor CDRH6D38 Shielded L ripple current. The LT1938’s switch current limit (I ) is CR75 Open LIM 1938fa 11

LT1938 APPLICATIONS INFORMATION Of course, such a simple design guide will not always re- ceramic input capacitor concerns the maximum input sult in the optimum inductor for your application. A larger voltage rating of the LT1938. A ceramic input capacitor value inductor provides a slightly higher maximum load combined with trace or cable inductance forms a high current and will reduce the output voltage ripple. If your quality (under damped) tank circuit. If the LT1938 circuit load is lower than 2A, then you can decrease the value of is plugged into a live supply, the input voltage can ring to the inductor and operate with higher ripple current. This twice its nominal value, possibly exceeding the LT1938’s allows you to use a physically smaller inductor, or one voltage rating. This situation is easily avoided (see the Hot with a lower DCR resulting in higher efficiency. There are Plugging Safety section). several graphs in the Typical Performance Characteristics For space sensitive applications, a 2.2µF ceramic capaci- section of this data sheet that show the maximum load tor can be used for local bypassing of the LT1938 input. current as a function of input voltage and inductor value However, the lower input capacitance will result in in- for several popular output voltages. Low inductance may creased input current ripple and input voltage ripple, and result in discontinuous mode operation, which is okay may couple noise into other circuitry. Also, the increased but further reduces maximum load current. For details of voltage ripple will raise the minimum operating voltage maximum output current and discontinuous mode opera- of the LT1938 to ~3.7V. tion, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (V /V > 0.5), there OUT IN Output Capacitor and Output Ripple is a minimum inductance required to avoid subharmonic The output capacitor has two essential functions. Along oscillations. See AN19. with the inductor, it filters the square wave generated by the Input Capacitor LT1938 to produce the DC output. In this role it determines the output ripple, and low impedance at the switching Bypass the input of the LT1938 circuit with a ceramic capaci- frequency is important. The second function is to store tor of X7R or X5R type. Y5V types have poor performance energy in order to satisfy transient loads and stabilize the over temperature and applied voltage, and should not be LT1938’s control loop. Ceramic capacitors have very low used. A 4.7µF to 10µF ceramic capacitor is adequate to equivalent series resistance (ESR) and provide the best bypass the LT1938 and will easily handle the ripple current. ripple performance. A good starting value is: Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has 100 C = high impedance, or there is significant inductance due to OUT V f OUT SW long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance where f is in MHz, and C is the recommended SW OUT electrolytic capacitor. output capacitance in µF. Use X5R or X7R types. This choice will provide low output ripple and good transient Step-down regulators draw current from the input sup- response. Transient performance can be improved with a ply in pulses with very fast rise and fall times. The input higher value capacitor if the compensation network is also capacitor is required to reduce the resulting voltage adjusted to maintain the loop bandwidth. A lower value ripple at the LT1938 and to force this very high frequency of output capacitor can be used to save space and cost switching current into a tight local loop, minimizing EMI. but transient performance will suffer. See the Frequency A 4.7µF capacitor is capable of this task, but only if it is Compensation section to choose an appropriate compen- placed close to the LT1938 and the catch diode (see the sation network. PCB Layout section). A second precaution regarding the 1938fa 12

LT1938 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMANDS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, POSCAP Tantalum Murata (408) 436-1300 www.murata.com Ceramic AVX www.avxcorp.com Ceramic, Tantalum TPS Series Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic When choosing a capacitor, look carefully through the Table 3. Diode Vendors data sheet to find out what the actual capacitance is under V I V AT 1A V AT 2A R AVE F F operating conditions (applied voltage and temperature). PART NUMBER (V) (A) (mV) (mV) A physically larger capacitor, or one with a higher voltage On Semiconductor MBRM120E 20 1 530 595 rating, may be required. High performance tantalum or Diodes Inc. electrolytic capacitors can be used for the output capacitor. B120 20 1 500 Low ESR is important, so choose one that is intended for B130 30 1 500 use in switching regulators. The ESR should be specified B220 20 2 500 B230 30 2 500 by the supplier, and should be 0.05Ω or less. Such a DFLS230L 30 2 500 capacitor will be larger than a ceramic capacitor and will International Rectifier have a larger capacitance, because the capacitor must be 10BQ030 30 1 420 470 20BQ030 30 2 470 large to achieve low ESR. Table 2 lists several capacitor vendors. Frequency Compensation Catch Diode The LT1938 uses current mode control to regulate the The catch diode conducts current only during switch off output. This simplifies loop compensation. In particular, the time. Average forward current in normal operation can LT1938 does not require the ESR of the output capacitor be calculated from: for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency I = I (V – V )/V D(AVG) OUT IN OUT IN compensation is provided by the components tied to the where I is the output load current. The only reason to V pin, as shown in Figure 2. Generally a capacitor (C ) OUT C C consider a diode with a larger current rating than necessary and a resistor (R ) in series to ground are used. In addi- C for nominal operation is for the worst-case condition of tion, there may be lower value capacitor in parallel. This shorted output. The diode current will then increase to the capacitor (C ) is not part of the loop compensation but F typical peak switch current. Peak reverse voltage is equal is used to filter noise at the switching frequency, and is to the regulator input voltage. Use a diode with a reverse required only if a phase-lead capacitor is used or if the voltage rating greater than the input voltage. Table 3 lists output capacitor has high ESR. several Schottky diodes and their manufacturers. 1938fa 13

LT1938 APPLICATIONS INFORMATION LT1938 VOUT = 12V, FRONT PAGE APPLICATION CURRENT MODE SW POWER STAGE OUTPUT gm = 3.5mho ERROR IL AMPLIFIER R1 CPL 1A/DIV FB – gm = 330µmho ESR 3M + 1.265V + C1 VOUT C1 100mV/DIV POLYMER CERAMIC VC GND OR TANTALUM 10µs/DIV 1938 F03 CF RC R2 Figure 3. Transient Load Response of the LT1938 Front Page CC Application as the Load Current is Stepped from 500mA to 1500mA. V = 3.3V OUT 1938 F02 may improve the transient response. Figure 3 shows the Figure 2. Model for Loop Response transient response when the load current is stepped from 500mA to 1500mA and back to 500mA. Loop compensation determines the stability and transient performance. Designing the compensation network is a BOOST and BIAS Pin Considerations bit complicated and the best values depend on the ap- plication and in particular the type of output capacitor. A Capacitor C3 and the internal boost Schottky diode (see practical approach is to start with one of the circuits in the Block Diagram) are used to generate a boost volt- this data sheet that is similar to your application and tune age that is higher than the input voltage. In most cases the compensation network to optimize the performance. a 0.22µF capacitor will work well. Figure 2 shows three Stability should then be checked across all operating ways to arrange the boost circuit. The BOOST pin must be conditions, including load current, input voltage and more than 2.3V above the SW pin for best efficiency. For temperature. The LT1375 data sheet contains a more outputs of 3V and above, the standard circuit (Figure 4a) thorough discussion of loop compensation and describes is best. For outputs between 2.8V and 3V, use a 1µF boost how to test the stability using a transient load. Figure 2 capacitor. A 2.5V output presents a special case because it shows an equivalent circuit for the LT1938 control loop. is marginally adequate to support the boosted drive stage The error amplifier is a transconductance amplifier with while using the internal boost diode. For reliable BOOST pin finite output impedance. The power section, consisting of operation with 2.5V outputs use a good external Schottky the modulator, power switch and inductor, is modeled as diode (such as the ON Semi MBR0540), and a 1µF boost a transconductance amplifier generating an output cur- capacitor (see Figure 4b). For lower output voltages the rent proportional to the voltage at the V pin. Note that boost diode can be tied to the input (Figure 4c), or to C the output capacitor integrates this current, and that the another supply greater than 2.8V. The circuit in Figure 4a capacitor on the V pin (C ) integrates the error ampli- is more efficient because the BOOST pin current and BIAS C C fier output current, resulting in two poles in the loop. In pin quiescent current comes from a lower voltage source. most cases a zero is required and comes from either the You must also be sure that the maximum voltage ratings output capacitor ESR or from a resistor R in series with of the BOOST and BIAS pins are not exceeded. C C . This simple model works well as long as the value C The minimum operating voltage of an LT1938 application of the inductor is not too high and the loop crossover is limited by the minimum input voltage (3.6V) and by the frequency is much lower than the switching frequency. maximum duty cycle as outlined in a previous section. For A phase lead capacitor (C ) across the feedback divider PL 1938fa 14

LT1938 APPLICATIONS INFORMATION VOUT 6.0 TO START BD 5.5 BOOST 5.0 VIN VIN LT1938 C3 V) E ( 4.5 G 4.7µF GND SW OLTA 4.0 T V TO RUN U 3.5 P N I 3.0 VOUT = 3.3V (4a) For VOUT > 2.8V 2.5 TLA = = 4 2.75µ°HC f = 800kHz 2.0 VOUT 0.001 0.01 0.1 1 10 LOAD CURRENT (A) BD D2 BOOST 8.0 VIN VIN LT1938 C3 TO START 7.0 SW 4.7µF GND E (V) 6.0 G A OLT 5.0 T V TO RUN U (4b) For 2.5V < VOUT < 2.8V NP 4.0 I VOUT = 5V VOUT 3.0 TA = 25° C L = 4.7µH BD f = 800kHz 2.0 BOOST 0.001 0.01 0.1 1 10 VIN VIN LT1938 C3 LOAD CURRENT (A) 1938 F05 SW 4.7µF GND Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit 1938 F04 voltage. In many cases the discharged output capacitor (4c) For VOUT < 2.5V will present a load to the switcher and the minimum input to start will be the same as the minimum input to run. Figure 4. Three Circuits For Generating The Boost Voltage This occurs, for example, if RUN/SS is asserted after V IN is applied. The plots show the worst-case situation where proper start-up, the minimum input voltage is also limited V is ramping very slowly. For lower start-up voltage, the by the boost circuit. If the input voltage is ramped slowly, IN boost diode can be tied to V ; however, this restricts the or the LT1938 is turned on with its RUN/SS pin when the IN input range to one-half of the absolute maximum rating output is already in regulation, then the boost capacitor of the BOOST pin. may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit At light loads, the inductor current becomes discontinu- will rely on some minimum load current to get the boost ous and the effective duty cycle can be very high. This circuit running properly. This minimum load will depend reduces the minimum input voltage to approximately on input and output voltages, and on the arrangement of 300mV above V . At higher load currents, the inductor OUT the boost circuit. The minimum load generally goes to current is continuous and the duty cycle is limited by the zero once the circuit has started. Figure 5 shows a plot maximum duty cycle of the LT1938, requiring a higher of minimum load to start and to run as a function of input input voltage to maintain regulation. 1938fa 15

LT1938 APPLICATIONS INFORMATION Soft-Start D4 MBRS140 The RUN/SS pin can be used to soft-start the LT1938, VIN VIN BOOST reducing the maximum input current during start-up. LT1938 The RUN/SS pin is driven through an external RC filter to RUN/SS SW VOUT create a voltage ramp at this pin. Figure 7 shows the start- VC up and shut-down waveforms with the soft-start circuit. GND FB By choosing a large RC time constant, the peak start-up BACKUP current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 20µA when the RUN/SS 1938 F07 pin reaches 2.3V. Figure 7. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT1938 Runs Only When the Input is Present IL LT1938 can pull large currents from the output through RUN 1A/DIV the SW pin and the V pin. Figure 7 shows a circuit that 15k IN will run only when the input voltage is present and that RUN/SS VRUN/SS 2V/DIV protects against a shorted or reversed input. 0.22µF GND VOUT 2V/DIV PCB Layout For proper operation and minimum EMI, care must be 2ms/DIV 1938 F06 taken during printed circuit board layout. Figure 8 shows the recommended component placement with trace, Figure 6. To Soft-Start the LT1938, Add a Resisitor and Capacitor to the RUN/SS Pin ground plane and via locations. Note that large, switched currents flow in the LT1938’s V and SW pins, the catch IN Shorted and Reversed Input Protection diode (D1) and the input capacitor (C1). The loop formed by these components should be as small as possible. These If the inductor is chosen so that it won’t saturate exces- components, along with the inductor and output capacitor, sively, an LT1938 buck regulator will tolerate a shorted should be placed on the same side of the circuit board, output. There is another situation to consider in systems and their connections should be made on that layer. Place where the output will be held high when the input to the a local, unbroken ground plane below these components. LT1938 is absent. This may occur in battery charging ap- The SW and BOOST nodes should be as small as possible. plications or in battery backup systems where a battery Finally, keep the FB and V nodes small so that the ground or some other supply is diode OR-ed with the LT1938’s C traces will shield them from the SW and BOOST nodes. output. If the VIN pin is allowed to float and the RUN/SS The Exposed Pad on the bottom of the package must be pin is held high (either by a logic signal or because it is soldered to ground so that the pad acts as a heat sink. To tied to V ), then the LT1938’s internal circuitry will pull IN keep thermal resistance low, extend the ground plane as its quiescent current through its SW pin. This is fine if much as possible, and add thermal vias under and near your system can tolerate a few mA in this state. If you the LT1938 to additional ground planes within the circuit ground the RUN/SS pin, the SW pin current will drop to board and on the bottom side. essentially zero. However, if the V pin is grounded while IN the output is held high, then parasitic diodes inside the 1938fa 16

LT1938 APPLICATIONS INFORMATION L1 C2 VOUT RRT CC RC R2 R1 D1 C1 GND RPG 1938 F08 VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VIN VIAS TO VOUT VIAS TO PG OUTLINE OF LOCAL GROUND PLANE Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation Hot Plugging Safely input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1µF capacitor improves high The small size, robustness and low impedance of ceramic frequency filtering. For high input voltages its impact on capacitors make them an attractive option for the input efficiency is minor, reducing efficiency by 1.5 percent for bypass capacitor of LT1938 circuits. However, these capaci- a 5V output at full load operating from 24V. tors can cause problems if the LT1938 is plugged into a live supply (see Linear Technology Application Note 88 for High Temperature Considerations a complete discussion). The low loss ceramic capacitor, combined with stray inductance in series with the power The PCB must provide heat sinking to keep the LT1938 source, forms an under damped tank circuit, and the cool. The Exposed Pad on the bottom of the package must voltage at the V pin of the LT1938 can ring to twice the be soldered to a ground plane. This ground should be tied IN nominal input voltage, possibly exceeding the LT1938’s to large copper layers below with thermal vias; these lay- rating and damaging the part. If the input supply is poorly ers will spread the heat dissipated by the LT1938. Place controlled or the user will be plugging the LT1938 into an additional vias can reduce thermal resistance further. With energized supply, the input network should be designed these steps, the thermal resistance from die (or junction) to prevent this overshoot. Figure 9 shows the waveforms to ambient can be reduced to θJA = 35°C/W or less. With that result when an LT1938 circuit is connected to a 24V 100 LFPM airflow, this resistance can fall by another 25%. supply through six feet of 24-gauge twisted pair. The Further increases in airflow will lead to lower thermal re- first plot is the response with a 4.7µF ceramic capacitor sistance. Because of the large output current capability of at the input. The input voltage rings as high as 50V and the LT1938, it is possible to dissipate enough heat to raise the input current peaks at 26A. A good solution is shown the junction temperature beyond the absolute maximum of in Figure 9b. A 0.7Ω resistor is added in series with the 125°C. When operating at high ambient temperatures, the 1938fa 17

LT1938 APPLICATIONS INFORMATION CLOSING SWITCH DANGER SIMULATES HOT PLUG VIN IIN VIN 20V/DIV LT1938 RINGING VIN MAY EXCEED + ABSOLUTE MAXIMUM RATING 4.7µF LOW STRAY IIN 10A/DIV IMPEDANCE INDUCTANCE ENERGIZED DUE TO 6 FEET 24V SUPPLY (2 METERS) OF 20µs/DIV TWISTED PAIR (9a) 0.7Ω VIN LT1938 20V/DIV + 0.1µF 4.7µF IIN 10A/DIV (9b) 20µs/DIV VIN LT1938 20V/DIV + + 22µF 35V 4.7µF AI.EI. IIN 10A/DIV (9c) 20µs/DIV 1938 F09 Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT1938 is Connected to a Live Supply maximum load current should be derated as the ambient Other Linear Technology Publications temperature approaches 125°C. Application Notes 19, 35 and 44 contain more detailed Power dissipation within the LT1938 can be estimated by descriptions and design information for buck regulators calculating the total power loss from an efficiency measure- and other switching regulators. The LT1376 data sheet ment and subtracting the catch diode loss and inductor has a more extensive discussion of output ripple, loop loss. The die temperature is calculated by multiplying the compensation and stability testing. Design Note 100 LT1938 power dissipation by the thermal resistance from shows how to generate a bipolar output supply using a junction to ambient. buck regulator. 1938fa 18

LT1938 TYPICAL APPLICATIONS 5V Step-Down Converter VIN V5VO UT 6.3V TO 25V 2.2A VIN BD ON OFF RUN/SS BOOST L 0.47µF 6.8µH 4.7µF VC LT1938 SW D RT 20k PG BIAS 590k 60.4k FB GND 680pF 200k 22µF f = 800kHz 1938 TA02 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter 4.4V TO 2V5IVN 3V.O3UVT 2.2A VIN BD ON OFF RUN/SS BOOST L 0.47µF 4.7µH 4.7µF VC LT1938 SW D RT 16.2k PG BIAS 324k 60.4k FB GND 680pF 200k 22µF f = 800kHz 1938 TA03 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB4R7M 1938fa 19

LT1938 TYPICAL APPLICATIONS 2.5V Step-Down Converter VIN 2V.O5UVT 4V TO 25V 2.2A VIN BD D2 ON OFF RUN/SS BOOST L 1µF 4.7µH VC SW 4.7µF LT1938 D1 RT 22.1k PG BIAS 196k 84.5k FB GND 680pF 200k 47µF f = 600kHz 3684 TA04 D1: DIODES INC. DFLS230L D2: MBR0540 L: TAIYO YUDEN NP06DZB4R7M 5V, 2MHz Step-Down Converter VIN V5VO UT 8.6V TO 22V 2A VIN BD ON OFF RUN/SS BOOST L 0.47µF 2.2µH 2.2µF VC LT1938 SW D RT 20k PG BIAS 590k 16.9k FB GND 680pF 200k 10µF f = 2MHz 1938 TA05 D: DIODES INC. DFLS230L L: SUMIDA CDRH4D22/HP-2R2 1938fa 20

LT1938 TYPICAL APPLICATIONS 12V Step-Down Converter VIN V12OVU T 15V TO 25V 2.2A VIN BD ON OFF RUN/SS BOOST L 0.47µF 10µH 10µF VC LT1938 SW D RT 30k PG BIAS 845k 60.4k FB GND 680pF 100k 22µF f = 800kHz 3684 TA06 D: DIODES INC. DFLS230L L: NEC/TOKIN PLC-0755-100 1938fa 21

LT1938 TYPICAL APPLICATIONS 1.8V Step-Down Converter VIN 1V.O8UVT 3.5V TO 25V 2.2A VIN BD ON OFF RUN/SS BOOST L 0.47µF 3.3µH 4.7µF VC LT1938 SW D RT 15.4k PG BIAS 84.5k 105k FB GND 680pF 200k 47µF f = 500kHz 1938 TA07 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB3R3M 1938fa 22

LT1938 PACKAgE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 ±0.05 3.50 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 0.38 ± 0.10 TYP 6 10 3.00 ±0.10 1.65 ± 0.10 (4 SIDES) (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 1 0.200 REF 0.75 ±0.05 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2.DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 1938fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. 23 However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LT1938 TYPICAL APPLICATION 1.265V Step-Down Converter VIN 1V.O2U6T5V 3.6V TO 25V 2.2A VIN BD ON OFF RUN/SS BOOST L 0.47µF 3.3µH 4.7µF VC LT1938 SW D RT 13k PG BIAS 105k FB GND 680pF 47µF f = 500kHz 1938 TA08 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1933 500mA (I ), 500kHz Step-Down Switching Regulator in V : 3.6V to 36V, V = 12V, I = 1.6mA, I < 1µA, ThinSOTTM OUT IN OUT(MIN) Q SD SOT-23 Package LT3437 60V, 400mA (I ), MicroPower Step-Down DC/DC V : 3.3V to 80V, V = 1.25V, I = 100µA, I < 1µA, DFN Package OUT IN OUT(MIN) Q SD Converter with Burst Mode Operation LT1936 36V, 1.4A (I ), 500kHz High Efficiency Step-Down V : 3.6V to 36V, V = 1.2V, I = 1.9mA, I < 1µA, MS8E Package OUT IN OUT(MIN) Q SD DC/DC Converter LT3493 36V, 1.2A (I ), 750kHz High Efficiency Step-Down V : 3.6V to 40V, V = 0.8V, I = 1.9mA, I < 1µA, DFN Package OUT IN OUT(MIN) Q SD DC/DC Converter LT1976/LT1977 60V, 1.2A (I ), 200kHz/500kHz, High Efficiency Step- V : 3.3V to 60V, V = 1.20V, I = 100µA, I < 1µA, TSSOP16E OUT IN OUT(MIN) Q SD Down DC/DC Converter with Burst Mode Operation Package LT1767 25V, 1.2A (I ), 1.1MHz, High Efficiency Step-Down V : 3V to 25V, V = 1.20V, I = 1mA, I < 6µA, MS8E Package OUT IN OUT(MIN) Q SD DC/DC Converter LT1940 Dual 25V, 1.4A (I ), 1.1MHz, High Efficiency Step-Down V : 3.6V to 25V, V = 1.20V, I = 3.8mA, I < 30µA, TSSOP16E OUT IN OUT(MIN) Q SD DC/DC Converter Package LT1766 60V, 1.2A (I ), 200kHz, High Efficiency Step-Down V : 5.5V to 60V, V = 1.20V, I = 2.5mA, I < 25µA, TSSOP16E OUT IN OUT(MIN) Q SD DC/DC Converter Package LT3434/LT3435 60V, 2.4A (I ), 200/500kHz, High Efficiency Step-Down V : 3.3V to 60V, V = 1.20V, I = 100µA, I < 1µA, TSSOP16E OUT IN OUT(MIN) Q SD DC/DC Converter with Burst Mode Operation Package LT3481 36V, 2A (I ), Micropower 2.8MHz, High Efficiency V : 3.6V to 36V, V = 1.265V, I = 5µA, I < 1µA, 3mm × 3mm OUT IN OUT(MIN) Q SD Step-Down DC/DC Converter DFN and MS10E Packages 1938fa 24 Linear Technology Corporation LT 1107 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com  LINEAR TECHNOLOGY CORPORATION 2007